Reformate is an aromatic product obtained by the catalyzed conversion of straight-run hydrocarbons boiling in the 70 to 190° C. range, such as straight-run naphtha. The reformate feedstock itself is obtained by fractionation or distillation of crude petroleum oil, its composition varying depending on the source of the crude oil, but generally having a low aromatics content. On conversion to reformate, the aromatics content is considerably increased and the resulting hydrocarbon mixture becomes highly desirable as a source of valuable chemical intermediates and as a component for gasoline. The principle components are a group of aromatics often referred to as BTX: benzene, toluene and the xylenes, including ethylbenzene. Other components may be present such as their hydrogenated homologues, e.g. cyclohexane.
Of the BTX group the most valuable components are benzene and the xylenes, and therefore BTX is often subjected to processing to increase the proportion of those two aromatics: hydrodealkylation of toluene to benzene and toluene disproportionation to benzene and xylenes. Within the xylenes, para-xylene is the most useful commodity. Xylene isomerisation or transalkylation processes have been developed to increase the proportion of para-xylene.
A further process that the gasoline producer can utilize is the hydrodealkylation of ethylbenzene to benzene.
Generally, the gasoline producer will isolate a fraction containing aromatic compounds containing at least 8 carbon atoms from the reformate stream, and then subject this stream to xylene isomerisation with the aim of maximising the para-xylene component. Xylene isomerisation is a catalytic process. Normally the para-xylene is then separated out to leave benzene, toluene (unless toluene conversion processes have already been applied), remaining mixed xylenes including ethylbenzene and aromatic compounds containing at least 9 carbon atoms. This alkylaromatic stream can be can be converted by (i) dealkylation to selectively eliminate ethylbenzene and to increase the yield of benzene while isomerizing xylenes to equilibrium or (ii) further reforming to convert ethylbenzene to xylenes while isomerizing xylenes to equilibrium or (iii) transalkylation by isomerizing xylenes to equilibrium and dealkylating specific alkylaromatic compounds. The latter process is the subject of the present invention.
U.S. Pat. No. 5,952,536 describes gas phase aromatics transalkylation with the help of a catalyst comprising a zeolite selected from the group consisting of SSZ-26, Al-SSZ-33, CIT-1, SSZ-35 and SSZ-44.
In transalkylation at this latter stage of the alkylaromatic treatment, it is advantageous if the catalyst is active in converting specific alkylaromatic compounds more especially methylethylbenzene, more preferably at relatively high xylene yield and/or relatively high total conversion. Furthermore, it is advantageous if the catalyst gives a product having a low ratio of ethylbenzene to total aromatic compounds having 8 carbon atoms. Furthermore, it is advantageous if the transalkylation catalyst is active, produces product which is high in xylene content, produces benzene of high purity, produces xylene of high purity, limits aromatics loss and/or isomerizes xylene to equilibrium. Aromatics may typically be lost by addition of hydrogen to form, for example, alkenes or alkanes.
Object of the present invention is to provide a catalyst which can be used in such process, a process in which such catalyst is used and a process for preparing such catalyst.